In packaging semiconductor chips, the principal technique is to mount the chips on ceramic or plastic substrates which are used to mount the chips and to provide the necessary pin input/output (I/O) connections. The substrates also may contain some circuit components such as capacitors and resistors. Many times it is desirable to package more than one kind of chip on a given substrate, or to make differnt types of connections to a single chip. Thus mounting pads for each of the chips must be provided as well as the necessary interconnect lines.
There are two principal systems for connecting chips to substrates. One system is the so called flip-chip solder bonding system, wherein solder connections on the face of the chip are soldered to solder pads formed on the substrate. This provides both mechanical bonding and electrical connections between the chip and substrate. The other system is a wire bonding system wherein wires, typically formed of aluminum or gold are connected such as by ultrasonic welding to aluminum, or gold, or other compatible metal on the chips and on the substrate. The chip may also be adhesively bonded to improve mechanical strength. Unfortunately, however, neither aluminium nor gold and solder are compatible, and they cannot be connected to each other, i.e., neither gold wire nor aluminum wire can be bonded to conventional Pb/Sn solder satisfactorily. Thus, it has not been possible to mix the two systems of connections on a single substrate, i.e., the multiple chip substrates have had to be all flip-chip solder bonded or all wire bonded, and any given chip would have only one type of connection.